The present invention relates to a mechanical device for transmitting power, and particularly to such a power-transmission device with a shear pin which is designed to fail at a predetermined force in order to protect automotive accessories such as a compressor of an automotive air conditioning system.
As is generally known, a shear pin is often provided in a fuse design to hold parts in a fixed relationship until excessive force or excessive torque is exerted on one or more of the parts which cause shearing of the shear pin, and also used as a safety device in a power train to protect a mechanism. One such a power-transmission device with shear pins has been disclosed in Japanese Utility Model Provisional Publication No. 1-169619. In the Japanese Utility Model Provisional Publication No. 1-169619, a power-transmission member is fixedly connected to a rotating member integrally connected to a shaft end of a rotating shaft of an automotive accessory through a plurality of shear pins, so that the shear pins are sheared when excessive load is exerted on the automotive accessory, in order for the shear pins to permit the power-transmission member to freely rotate and thus to prevent the automotive accessory to be damaged. As shear pins functioning as a simple and reliable overload limiter, a shearable connecting structure constructed by notched bolts and nuts is often used. Each of the notched bolts has a circumferentially notched portion at coupled faces of the rotating member and the power-transmission member. The notched bolts serve as shear pins. When assembling, the nuts are screwed onto the respective notched bolts to add a joining force in an axial direction of each of the bolts and to couple the rotating member with the power-transmission member by means of the notched bolts and nuts. For easy assembling of the rotating member and power-transmission member, a bolt insertion hole machined in each of these members must be dimensioned to be somewhat larger than an outside diameter of the external threaded portion of each of the notched bolts. However, when the notched bolts are inserted into the respective bolt insertion holes, each having an inside diameter larger than the outside diameter of the notched bolt, and then the rotating member and the power-transmission member are assembled to each other by means of the notched bolts and nuts, there is a considerable dimensional divergence in an aperture among each set of notched bolt and associated bolt insertion hole, which aperture dimension is measured in a direction of rotation of the rotating member and power-transmission member. In presence of excessive load applied to the accessory, there is an increased tendency for a shearing force to be exerted ununiformly on the notched portion of each bolt. In the power-transmission device as described in the Japanese Utility Model Provisional Publication No. 1-169619, owing to unbalanced load arising from the ununiform shearing forces applied to the notched portions of the bolts, an intended shearing performance, that is, an intended torque-limiter performance cannot be assured. Additionally, if the notched bolt serving as a shear pin and the associated nut are tightened with excessive torque greater than a specified tightening torque, the excessive tightening torque acts to prevent the rotating member from displacing relative to the power-transmission member in the rotational direction when excessive load is applied to the automotive accessory. The tightening forces acting in the axial directions of the bolts also act on the respective notched portions, thus exerting a bad influence on the intended shearing performance.
Accordingly, it is an object of the invention to provide a mechanical device for transmitting power, which avoids the aforementioned disadvantages.
It is another object of the invention to provide a shear-pin equipped power-transmission device for use in a power train of an automotive accessory such as an air-conditioning compressor, which is capable of ensuring easy assembly without sacrificing an intended shearing performance of the shear pin.
In order to accomplish the aforementioned and other objects of the present invention, a mechanical device for transmitting power comprises a power-transmission member rotatably mounted on a housing of an automotive accessory so that the power-transmission member is arranged concentrically with respect to a rotating shaft of the automotive accessory, a rotating member fixedly connected to a shaft end of the rotating shaft and arranged to be opposite to the power-transmission member in an axial direction of the rotating shaft, a plurality of shear pins via which the power-transmission member and the rotating member are joined to each other, a first member of the power-transmission member and the rotating member, to which the shear pins are fixedly connected, a second member of the power-transmission member and the rotating member having a portion defining a plurality of shear-pin insertion holes, an inside diameter of each of the shear-pin insertion holes being dimensioned to be larger than an outside diameter of each of the shear pins, each of the shear pins having a notched portion between opposing faces of the power-transmission member and the rotating member, and a flanged portion formed on an outer periphery thereof at a position being offset from the notched portion toward the second member and interleaved between the power-transmission member and the rotating member so that the notched portion adjoins the flanged portion and that both side walls of the flanged portion abut the opposing faces of the power-transmission member and the rotating member, and the second member being engaged with the shear pins through the shear-pin insertion holes, and fixedly connected to the shear pins by applying a tightening force in an axial direction of each of the shear pins between a protruded end portion of each of the shear pins projecting outside of the shear-pin insertion holes and an associated one of the flanged portions of the shear pins. It is preferable that each of the shear pins has an external threaded portion formed on the protruded end portion, so that the second member is fixedly connected to the first member via the shear pins by tightening a nut screwed onto the external threaded portion. Alternatively, the second member may be fixedly connected to the first member via the shear pins by caulking the protruded end portion of each of the shear pins. The rotating member may comprise a drive plate fixedly connected to the shaft end of the rotating shaft of the automotive accessory, and the drive plate may comprise a first plate fixed to the shaft end of the rotating shaft, a second plate located on an outer periphery of the first plate and integrally connected to the power-transmission member via the shear pins, and a damper rubber through which the first and second plates are integrally connected to each other. It is more preferable that the flanged portion is cylindrical in shape and the notched portion is frusto-conical in shape so that the smallest-diameter portion of the frusto-conical notched portion adjoins one side wall of the flanged portion facing the first member. Preferably, the damper rubber has a substantially ring shape. The damper rubber has a radially inwardly extending brim or flange portion and a radially outwardly extending brim or flange portion. The radially inwardly extending flange portion of the ring-shaped damper rubber is partly provided in an aperture defined between opposing faces of the first plate and the first member, whereas the radially outwardly extending flange portion of the ring-shaped damper rubber is partly provided in an aperture defined between opposing faces of the second plate and the first member, and whereby the second member is in elastic-contact with the first member via the flange portions of the ring-shaped damper rubber, for power-train torque-shock reduction.
The other objects and features of this invention wil become understood from the following description with reference to the accompanying drawings.